140 



KOHN 



ridges and the bottoms of depressions is only about 1 ft 

 (30 cm). As on the coral-algal ridge zone, sheets of yellow 

 Acropora and Millepora are conspicuous. In addition there 

 arc scattered low, rounded heads of Porites lobata and sev- 

 eral species of faviids ...living coral colonies on these low 

 heads are often crescent or doughnut shaped probably 

 because the higher center portions are killed periodically 

 by exposure during exceptionally low spring tides. Filamen- 

 tous red, brown, green, and blue-green algae form heavy 

 encrusting mats over all of the zone which is not covered 

 by coral, there being no areas of white sand as in the back 

 reef zones. Small sea anemones are abundant, occurring in 

 clusters throughout the algal mat. ...Coral cover is much 

 less than half the surface area. ...The zone receives pulses 

 of foam-water as the breakers throw rolls of water up on 

 the plateau. Since there is a distinct slope the current is 

 always strong even at low spring tide when the water 

 pours steadily across like a broad mountain stream rippling 

 over a rocky bed" (Odum and Odum, 1955). 



4. Zone of smaller heads, 5. Zone of larger heads, and 

 6. Zone of sand and shingle are the lagoonward zones dis- 

 tinguished by Odum and Odum (1955). They are com- 

 pletely subtidai and will not be further discussed here. 



The following description of a windward reef platform 

 adjacent to an island is taken from Leviten and Kohn 

 (1980). The platform on the windward (east) side of 

 Enewetak Island (Fig. 1) is about 90 to 120 m wide, "and 

 mainly topographically simple or smooth, but certain por- 

 tions have numerous holes and depressions. Wave action is 

 extremely heavy and constant on the seaward bench mar- 

 gin at high tide, but is damped somewhat on the inner por- 

 tions by an extensive low coralline algal ridge en the sea- 

 ward margin." At Sta. F7 of Leviten and Kohn (1980), 

 "the inner 20 m is scalloped and pitted reef rock undergo- 

 ing chemical dissolution (Revelle and Emery, 1957). It 

 lacks macroscopic algal cover, but in certain areas bears a 

 thin, slippery film of blue-green algae [Calothrix 

 Crustacea]. ...An incipient algal turf begins —25 m from 

 shore and increases in luxuriance in a seaward 

 direction. ...A shallow swale, unique to this bench among 

 those studied, occurs between 50 and 65 m from shore. 

 This area is covered by several centimeters of water even 

 during low tide and harbors a healthy 2-cm-thick algal 

 turf. ...The bench is pitted and dissected between 65 and 

 80 m from shore. Algal cover is thick, and algal species 

 richness is higher than on other portions of the 

 bench. ...The Porolithon ridge is evident ~80 m from 

 shore and continues to the seaward margin of the bench 

 ~100 m from shore, where it is dissected by numerous 

 small surge channels and grooves. The ridge has a scoured 

 aspect, possibly due to intense grazing activity by herbivo- 

 rous fishes at high tide, and lacks an extensive fleshy algal 

 cover, save for a film of blue-green algae." 



In order to characterize the microhabitats of benthic 

 invertebrates more precisely, bench substrates are classi- 

 fied as shown in Table 1 (see also Leviten and Kohn, 

 1980). 





;7V 



^- *' ^ ' - 



•"**-■ 



'-?*. 



ia, 



r^rer- 





■-a^S^ 



Fig. 1 The windward platform of Enewetak Island, a, look- 

 ing NE at a neap low tide. April, 1979; b, a 1-m^ quadrat on 

 the same area shown in a, indicating the smooth topography 

 of the substrate. 



Temperatures recorded by Havens (1974) on intertidal 

 substratum exposed to air reach 36.5°C on exposed reef 

 rock on the windward reef platform, 38.5°C on bench 

 rock, and 39°C on rubble-covered beaches. The maximum 

 temperature recorded under rocks and in holes was only 

 32.5°C. Water temperatures in tidal pools reach 38°C, in 

 contrast to submerged reefs, which do not exceed 32°C. 



GEOLOGICAL PERSPECTIVE 



Shallow-water marine environments at Enewetak 

 extend back in time 50,000,000 years. A vertical borehole 

 drilled in 1952 (Ladd and Schlanger, 1960) reached vol- 

 canic bedrock after traversing 1300 m of calcareous 

 material derived solely from shallow reef-building and reef- 

 associated organisms. Fossils in the reef limestone just 

 above the discontinuity were of Eocene age, indicating a 

 long-term average su'^sioence rate of about 0.03 mm yr 

 (Menard, 1964). Reco.ery of land mollusc fossils from core 

 samples indicates that, during probably cooler parts of 

 the Neogcne, Enewetak stood higher above sea level than 

 at the present time (Ladd, 1958). 



